(1) Field of the Invention
The present invention relates to an improvement of non-aqueous electrolyte cells for the purpose of improving the high-temperature cycle characteristics.
(2) Description of the Prior Art
In recent years, there has been a rapid reduction in the size and weight of mobile information terminals such as mobile telephones and notebook personal computers. Under the circumstances, much attention is being paid to non-aqueous electrolyte cells that utilize gelled non-aqueous electrolytes (polymer electrolytes).
The non-aqueous electrolyte cells use a gelled non-aqueous electrolyte for the electrolyte and hence do not suffer solution leakage, making it possible to use a more light-weight outer casing body such as an aluminum laminate film. Such cells realize a significant reduction in weight compared with known cells using metal cans.
The non-aqueous electrolyte cells use a prepolymer electrolyte that is a mixture of a non-aqueous solvent, an electrolyte salt, and a polyether-based monomer (prepolymer). The polyether-based monomer has at its molecular edge a polymerizable functional group such as acrylate and methacrylate. The cells are produced by polymerizing the prepolymer within the cell outer casing bodies with the use of heat, UV rays, a polymerization initiator, or the like. The non-aqueous electrolyte cells thus obtained excel in ion conductivity.
However, the non-aqueous solvent contained in the gelled non-aqueous electrolyte is problematic in that it is reduced and decomposed at the negative electrode, and thus causes gas generation. With the progress of the charge and discharge cycles, the gas accumulates between the electrode and the electrolyte and thus degrades the contactivity thereof. This increases internal resistance, resulting in degraded cell cycle characteristics.
Known techniques to inhibit the degradation of the cycle characteristics attributed to the above reason include adding vinylene carbonate derivatives in the electrolyte. According to this technique, the vinylene carbonate reacts with the negative electrode and forms a stable coating film thereon, which inhibits the reaction of the negative electrode and the non-aqueous electrolyte. This is claimed to reduce gas generation. However, this technique presents another problem; the vinylene carbonate is oxidized and decomposed at the positive electrode, again resulting in gas generation.
Techniques to enhance the ion conductivity of a high-molecular solid electrolyte include one disclosed by patent document 1. This is a technique of blending a metal salt of Ia group of the periodic table in a polymer of polyester (meth)acrylate in which at least part of the hydroxyl groups of polyester polyol is converted into (meth)acrylic ester. Patent document 1 thus tries to obtain a flexible high-molecular solid electrolyte that has high ion conductivity and electrochemical stability.
Patent document 1: Japanese Unexamined Patent Publication No. 2000-311516 (Abstract)
However, even the high-molecular solid electrolyte obtained by this technique is reduced and decomposed at the negative electrode when temperature becomes high, which still causes gas generation. This reduces the contactivity of the electrode and the electrolyte, resulting in degraded cycle characteristics. Thus, this technique is still problematic.